1. Field of the Invention
The present invention relates to an active-matrix bistable display device capable of reducing the number of signal line amplifiers (H (horizontal) drivers).
The present application claims priority of Japanese Patent Application No. 2005-101750 filed on Mar. 31, 2005, which is hereby incorporated by reference.
2. Description of the Related Art
In recent years, a bistable display device is being developed as a device to be used in a display section or a like of electronic papers, public displays, and ICs (Integrated Circuit). The bistable display is used mainly as a reflective display device which has a characteristic in that reduction of power consumption can be achieved easily since an image signal is input only at time of display rewriting and the image signal is not input at a time of no rewriting.
Examples of the bistable display device include an electrophoretic display device (EDP) [see Non-Patent Reference 1: SID (Society of Information Display) 04, Digest p. 133], polymer network liquid crystal display (see Non-Patent Reference 2: Next Generation Liquid Crystal Display, Kyoritsu Publishing Co., p. 57), and bistable nematic liquid crystal display device (see Non-Patent Reference 3: Next Generation Liquid Crystal Display, Kyoritsu Publishing Co., p. 1), or a like. Of these display devices, the electrophoretic display device is assumed to be most promising since it has a simplified-structure and can be fabricated at low costs and consumes less power and is excellent in stability of displaying.
The electrophoretic display device is so configured that a transparent surface plate having a facing electrode made up of transparent conductive films in its inner face and pixel electrode plates in which pixel electrodes are arranged in a row direction and in a column direction are placed at a short interval and a toner powder obtained by mixing two kinds of charged particles each having a different polarity is hermetically sealed in gap space between the surface plate and each of the pixel electrode plates.
In such the electrophoretic display device as above, ordinarily, when a pixel electrode is made to be at a plus (+) potential by making a facing electrode be at a 0 (zero) potential and by controlling a voltage to be applied to a pixel electrode, black particles each having a positively-charged polarity are attracted toward the facing electrode side and white particles each having a negatively-charged polarity are attracted toward the pixel electrode side and, as a result, black is displayed through a transparent surface plate, whereas, when a pixel electrode is made to be at a minus (−) potential, white particles are attracted toward the facing electrode side and black particles are attracted toward the pixel electrode side and, as a result, white is displayed on the surface plate side. Thus, by controlling a polarity of a voltage to be applied to every pixel electrode, a character, image, or a like can be displayed.
Some of the electrophoretic display devices are so constructed that positively-charged black particles and negatively-charged white particles are sealed hermetically in a micro capsule and so as to have a film shape. In the case of this type of electrophoretic display device, to display black, when a voltage is applied, the black particles in the micro capsule are attracted toward a facing electrode and the white particles in the micro capsule are attracted toward a pixel electrode, whereas, to display white, when a voltage is applied, the white particles in the micro capsules are attracted toward the facing electrode and the black particles in the micro capsule are attracted toward the pixel electrode and, as a result, a character and/or image are displayed as in the case described above.
FIG. 18 is a graph for showing an example of a display characteristic of the electrophoretic display device. In any type of the electrophoretic display device, a concentration of black becomes high as a plus (+) voltage to be applied to a pixel electrode becomes high and a concentration of white becomes high, as a minus (−) voltage to be applied to a pixel electrode becomes high and, in either direction, as a voltage becomes high, a concentration comes near to a saturated state (100%) and becomes stable, which provides bistability in displaying. Such the distribution state of black and white is held even when a voltage of a pixel electrode is 0 (zero) V or the pixel electrode is opened, which provides memory property in displaying.
Moreover, pixel electrodes have a plurality of scanning lines extending in a row direction and a plurality of signal lines extending in a column direction in its lower location and have a TFT (Thin Film Transistor) substrate made up of TFT transistors in which a driving transistor is formed at each intersection of each of the scanning lines and each of the signal lines. Each of the pixel electrodes is of an active-matrix type in which, when a corresponding scanning line is driven, each of the TFTs becomes active, resulting in each of the pixel electrodes being connected to a corresponding signal line and a voltage of the signal line being applied to each of the pixel electrodes.
FIG. 19 is a schematic diagram showing configurations of a display panel used when a display section of a conventional electrophoretic display device is driven in a active mode. In the conventional electrophoretic display device, as shown in FIG. 19, a plurality of signal lines D1, D2, . . . , Dn, Dn+1, . . . extending in a column direction and a plurality of scanning lines G1, G2, . . . , Gm, Gm+1, . . . extending in a direction orthogonal to the column direction are provided and each of TFTs [(T1.1, T2.1, . . . , Tn.1, T(n+1).1, . . . ), (T1.2, T2.2, . . . , Tn.2, T(n+1).2, . . . ), (T1.m, T2.m, . . . , Tn.m, T(n+1).m, . . . ), (T1.(m+1), T2.(m+1), . . . , Tn.(m+1), T(n+1).(m+1), . . . ), . . . ], which are made of amorphous silicon (a-Si) or a like, is formed at each intersection of each of the signal lines D1, D2, . . . , Dn, Dn+1, . . . and each of the scanning lines G1, G2, . . . , Gm, Gm+1, . . . and, when driving of each of the signal lines coincides with that of each of the scanning lines, each of the TFTs connected at each intersection of each of the signal lines and each of the scanning lines becomes active and switching is done so that a voltage of each of the signal lines is applied to each of pixel capacitors [C1.1, C2.1, . . . , Cn.1, C(n+1).1, . . . ), (C1.2, C2.2, . . . , Cn.2, C(n+1).2, . . . ), (C1.m, C2.m, . . . , Cn.m, C(n+1).m, . . . ), (C1(m+1), C2.(m+1), . . . Cn.(m+1), C(n+1).(m+1) . . . ].
Each of the pixel capacitors represents the capacitor formed between each of the pixel electrodes connected to each of corresponding TFTs shown in an upper portion in FIG. 5 and a facing electrode (not shown) whose connecting state is indicated by each of circular marks shown in a lower portion in FIG. 5.
FIGS. 20A and 20B show a difference in driving methods between an ordinary liquid crystal display device and a bistable display device. In each of the ordinary liquid crystal display devices, as shown in FIG. 20A, when a scanning signal is at an ON voltage by a scanning signal applied to each of scanning lines and an image signal input to each of the signal lines, a corresponding TFT is turned ON and the image signal of each of the signal lines is written to each of pixel capacitors and, after the scanning signal is turned OFF, the image signal is held in each of the pixel capacitors for one frame and, as a result, an image is displayed. Then, after the operation of displaying an image has been complete, by lowering a voltage of the signal line, the image being displayed is erased.
On the other hand, the bistable display device generally provides a response speed of as slow as about 100 ms to 1000 ms and has a memory property (image holding property). Therefore, for example, presuming that one frame is 1/60 sec., as shown in FIG. 20B, ordinarily, after the same voltages are applied for writing in a plurality of frames during an image signal writing period, during an image holding period, no voltage is applied or the voltage is made to be 0V. Then, at time of termination of the image holding period, by applying a voltage of opposite polarity during an image erasing period made up of the plurality of frame periods, an image that had been displayed is erased.
In the bistable display device, unlike in the case of the ordinary liquid crystal display, generally requires no highly-accurate gap control, however, since a distance between a pixel electrode and a facing electrode is large, it is necessary that an image signal voltage at time of writing is made higher. In the bistable display device having a film structure in particular, a thickness of a film is about 100 μm which is considerably long when compared with the case of the liquid crystal display and, as a result, the distance between the pixel electrode and facing electrode is large and it is, therefore, necessary that an image signal voltage at time of writing driving is higher. Due to this, there is a problem that a signal line driver (H (horizontal) driver) to drive a signal line requires a highly withstand process and it is necessary that a data register, latch, D/A (digital to analog) converter or a like are built into the signal line driver, which causes costs for manufacturing the signal line driver to become higher when compared with a scanning driver (V (vertical) driver) made up of only shift registers.
To solve this problem, in order to reduce the number of horizontal drivers in the active-matrix display device, a double-speed driving method is disclosed in Patent Reference 1 (Japanese Patent Application No. Hei03-038689) and Patent Reference 2 (Japanese Patent Application No. Hei04-360127) in which the number of scanning lines is doubled and the number of signal lines is reduced to one-half. In this case, by connecting two pieces of pixels to each signal line through each TFT and gates of two pieces of TFTs to each of different scanning lines, selection of signals to be applied for writing to the two pieces of pixels is made possible. Therefore, for example, in the case of a VGA (Video Graphics Array)-type liquid crystal display, the number of scanning lines is increased to be 480×2=960, however, the number of signal lines is decreased to be 1920/2=960. By configuring as above, when compared with the conventional display device, though the number of vertical drivers increases, the number of highly-priced horizontal drivers decreases, thus enabling reduction of costs of manufacturing the active-matrix display device. However, the technologies disclosed in the Patent References 1 and 2 are to be applied to the ordinary liquid crystal display device providing no bistability and cannot be applied to the bistable display device of the present invention.
A display device using a cholesteric liquid crystal is disclosed as a bistable display device in the Patent Reference 3. It is known that the cholesteric liquid crystal display device, though differing in its characteristics from the electrophoretic display device, provides bistability for displaying. However, the technology disclosed in the Patent Reference 3 is to be applied to a passive-matrix display device providing no bistability for displaying and cannot be applied to the bistable display device of the present invention.
Thus, it is not conventionally known that configurations in which the number of highly-priced horizontal drivers can be reduced in the active-matrix bistable display device.